Process for producing a superconducting cable

ABSTRACT

A process for producing superconducting cable consisting of at least one elongated superconducting element, containing a cable core, and a flexible tube enclosing the cable core, is described, this process comprising the following steps:  
     (a) the cable core is drawn continuously from a supply unit;  
     (b) a metal strip is drawn continuously from a strip supply unit;  
     (c) the metal strip is shaped continuously around the cable core to form a slotted tube; the longitudinal slot is welded; and the welded tube is then corrugated, the inside diameter of the corrugated tube being greater than the outside diameter of the cable core;  
     (d) the superconducting cable consisting of the cable core and the corrugated tube is wound up on a cable drum, or at least one turn of the superconducting cable has been run; and  
     (e) finally, the ends of the cable core are joined mechanically to the ends of the corrugated tube while the cable is on the cable drum or after at least one turn of the cable has been run.

[0001] The invention pertains to a process for producing asuperconducting cable.

[0002] A flexible, high-power, DC superconducting cable is known fromElektrotechnische Zeitung (ETZ-b), Vol. 26, p. 215, 1974. This cable hasa ribbon-like superconductor made of stabilized Nb₃Sn, which issurrounded by a so-called “cryogenic envelope”.

[0003] The conductor is hollow, and liquid helium flows through itduring operation. Between the cryogenic envelope and the conductor,there is a channel, through which liquid or gaseous helium also flows asa return line.

[0004] The cryogenic envelope consists of several corrugated metaltubes, arranged a certain distance from each other, between which avacuum is present as well as so-called “superinsulation”.

[0005] A cable of this type can be produced in lengths of up to 200 m ona continuous production basis. The maximum transmission-line power of asystem consisting of two of these types of single-conductor cables is 5GW.

[0006] A superconducting cable in which a plurality of tapes, consistingof so-called 2nd-generation superconducting material, are wrapped arounda support tube, through which liquid nitrogen flows, is known fromspring 1999 edition of the Epri Journal. These modern superconductorsconsist of flexible metal tape, onto which an yttrium-barium-copperoxide compound has been applied.

[0007] A corrugated metal tube is provided over the superconductingtapes. Several layers of thermal insulation are then wrapped around thecorrugated metal tube. The thermal insulation is surrounded by a secondcorrugated metal tube, and a spacer of Teflon is also provided on thethermal insulation. A solid dielectric and an external electrical shieldare installed around the second corrugated metal tube.

[0008] Common to both types of cable is that, when the cable is in theoperating state and the superconductor and the cable elementssurrounding it are at the temperature of the liquid helium or of theliquid nitrogen, they become shorter than the external cable elements,which are at ambient temperature. To prevent this decrease in length,fittings are provided at both ends of the cable to connect theindividual cable elements nonpositively to each other and thus toprevent the inner cable elements from becoming shorter than the outercable elements.

[0009] Proceeding from this state of the art, the task of the presentinvention consists in providing a process for the production ofsuperconducting cable, according to which it is guaranteed during theproduction process itself that the cable core will not contract as aresult of a drop in temperature and become shorter than the outer cableelements, which are at ambient temperature.

[0010] This task is accomplished by the characterizing features statedin claim 1.

[0011] Because of the way the cable core is designed, it is much stifferthan the corrugated tube. The cable core therefore lies against theinside surface of the radially outer part of the wall of the corrugatedtube; that is, the turns of the cable core try to assume the largestpossible diameter. Because, according to the invention, the insidediameter of the corrugated tube is larger than the outside diameter ofthe cable core, the length of the cable core located inside thecorrugated tube is greater, relative to the center axis in question,than the length of the corrugated tube. When the ends of the cable coreare now connected to the ends of the corrugated tube while thesuperconducting cable is on the cable drum, there is an excess length ofthe cable core in the corrugated tube. When the superconducting cable isunwound later from the cable drum, either the corrugated tube stretchesby an amount which corresponds to this excess, or the cable core comesto rest at intervals against the inside wall of the corrugated tube in awave-like manner. When the superconducting cable is operating, i.e.,when it is at the temperature of liquid nitrogen, the increase in thelength of the corrugated tube is reversed or the wavy form of the cablecore is lost.

[0012] The invention is explained in greater detail on the basis of anexemplary embodiment, which is illustrated schematically in the figures.

[0013]FIG. 1 shows a lateral cross section through a superconductingcable, which has been produced according to the principle of theinvention. The cable has a core 1, which consists, in a manner not shownhere, of an inner support tube; one or more layers of high-temperaturesuperconducting material, which are wrapped around the support tube; anda dielectric, which is applied over the layers of high-temperaturesuperconducting material. The dielectric must have high cold resistanceand can consist, for example, of continuous filament glass cloth, micatapes, or possibly polytetrafluoroethylene.

[0014] The cable core 1 is located in a so-called “cryogenic envelope”,which is built up out of an inner corrugated tube 2 and an outercorrugated tube 3, which is located a certain distance away from theinner corrugated tube 2.

[0015] A layer 4 is wound around the inner corrugated tube; this layerconsists of plastic film, both sides of which have been coated withvapor-deposited aluminum. A spacer 5 is wound around the layer 4; thisspacer consists advantageously of several strands of plastic, glassfibers, or a ceramic material, which are interwoven with each other in abraid-like manner.

[0016] A so-called “superinsulating” layer 6 surrounds the spacer 5.This layer consists of alternating plies of metal foils or metal-coatedplastic films and sheets of nonwoven plastic material or of a nonwovenmaterial based on glass fibers or ceramic materials.

[0017] The structure of a cryogenic envelope is the object of EP 0 326923 B 1.

[0018] Before the superconducting cable is put into service, thering-shaped space 7 between the corrugated tubes 2 and 3 is evacuated.

[0019] To cool the superconducting cable 1, liquid nitrogen is conductedthrough the support tube of the cable core and through the interiorspace of the corrugated tube 2.

[0020]FIG. 2 shows a side view of a production system, by means of whichthe superconducting cable can be fabricated.

[0021] A metal strip 11 is pulled continuously from a coil 10; thisstrip is cleaned if desired in a strip-cleaning unit 12 and is graduallyformed in a shaping device 13 into a slotted tube with a longitudinalseam.

[0022] The longitudinal seam of the slotted tube is then welded in awelding station 16, and the welded tube with the cable core 14 inside itis sent by pay-off device 17 to a corrugating device 18.

[0023] In the corrugating device 18, the longitudinally welded tube iscorrugated, and then it is wound up on a cable drum 19. A so-calleddancer 20 regulates the pay-off speed of the cable drum 19.

[0024] After the desired length of superconducting cable has beenproduced or after the cable drum 19 has become full, the two ends of thecable core 14 are connected nonpositively to the ends of the corrugatedtube 2.

[0025] Because the cable core 14 is much stiffer than the corrugatedtube, the cable core 14 rests against the corrugated tube, namely,against the larger radius of the coiled corrugated tube. The cable coreinside the corrugated tube is therefore longer than the corrugated tube.

[0026] In a following step of the process, the elements shown in FIG. 1,namely, the metal foil 4, the spacer 5, the superinsulation 6, and theouter corrugated tube 3, are applied over the corrugated tube in asimilar type of production system.

[0027]FIG. 3 shows a cross section through the core of the cable drum19, on which the corrugated metal tube (inner corrugated tube 2according to FIG. 1) with the cable core 1 or 14 (according to FIG. 2)inside it is wound up. It is easy to see that the stiff cable core 1rests against the radially outer part of the inside surface of thecorrugated tube 2, and thus the length which it has inside thecorrugated tube 2 is greater than the length of the corrugated tube 2.

[0028] The length difference Δ1 (=excess length) is calculated by theuse of the following formula:

Δl=Δr×π×2×a

[0029] where Δr is the difference between the radii of the insidediameter of the corrugated tube 2 and the outside diameter of the cablecore of the superconducting cable on the drum, and a is the number ofturns of cable on the cable drum.

[0030] If the corrugated tube 2 has an inside diameter of 60 mm, thecable core has an outside diameter of 50 mm, and the core diameter is3,000 mm, we obtain an excess length of 3 cm per turn. The total lengthof one turn of the tube is 9.4 m. The excess length is thus 0.3%. Thus acontraction of the cable core of less than 0.3% as the core cools downto the temperature of liquid nitrogen can therefore be easilycompensated.

[0031]FIG. 3 shows a length of cable which has been wound around a cabledrum 19. It can be seen easily here that the cable core 1, because ofits stiffness, rests against the radially outside wall of the corrugatedtube 2.

[0032] The cable core 1 and the corrugated tube 2 are connectednonpositively to each other while they are still wound around the drum,as indicated schematically at 21 and 22.

1-4. (Cancelled).
 5. Process for the production of a superconductingcable having a cable core, which contains at least one elongatedsuperconducting element, and a flexible tube, which surrounds the cablecore, said process comprising the steps of: (a) the cable core is pulledcontinuously from a supply unit; (b) a metal strip is pulledcontinuously from a strip supply unit; (c) the metal strip is formedcontinuously around the cable core to form a slotted tube; thelongitudinal slot is welded shut; and then the welded tube iscorrugated, where the inside diameter of the corrugated tube is largerthan the outside diameter of the cable core; (d) the superconductingcable consisting of the cable core and the corrugated tube is wound upon a cable drum, or the superconducting cable is laid to form at leastone turn; and (e) the ends of the cable core are then mechanicallyjoined to the ends of the corrugated tube while the cable is on thecable drum or is lying in at least one turn.
 6. Process according toclaim 5, wherein the forming of the metal strip into a tube or thecorrugation of the welded metal tube is carried out in such a way thatthe cable core has an excess length Δl in the corrugated metal tube,which is calculated according to the formula Δl=(R−r)π×2a, where R isthe inner radius of the corrugated tube, r the outer radius of the cablecore, and a the number of turns.
 7. Process according to claim 5,wherein the excess length Al is at least 0.25%.
 8. Process according toclaim 5, wherein the cable core contains a high-temperaturesuperconductor.